The invention relates to an apparatus and a method for folding print sheets or signatures.
For the further processing of printed-on sheets of paper, so-called print sheets which comprise two or more printed pages, these print sheets are bent over at least once under the effect of pressure to form a sharp edge, meaning they are folded to the format of the finished printed product along a previously perforated or grooved folding line, or also along a non-prepared folding line, according to a predetermined folding pattern. With reference to the feeding direction for the print sheets, cross-folding as well as longitudinal folding devices are used for this, wherein a single device, multiple devices or also a combination thereof can be arranged in a folding machine. The folding can be realized with the aid of a so-called pocket folding and/or sword folding principle. Regardless of the type of apparatus used and the respective method, the precision of each individual fold is decisive for the quality of the printed product to be produced.
German patent document DE 3544495 A1 discloses a folding apparatus for the cross-folding which operates based on the sword-folding principle. With this apparatus, respectively one print sheet is supplied with the aid of feed rollers in a feeding direction to a folding table and against an end stop arranged thereon. The folding table contains an opening, embodied transverse to the feeding direction, above which a mechanically operated folding sword is arranged. Two folding rollers are arranged below the folding table, in the region of and parallel to this opening. As soon as the front edge of the print sheet to be folded impacts with an end stop that projects upward from the folding table, as seen in the feeding direction, this triggers the lowering of the folding sword onto the print sheet positioned on the folding table. The folding sword presses against the folding line, meaning against an imaginary straight line on the print sheet along which the sheet is to be folded, and thus moves the print sheet through the opening of the folding table and toward the folding rollers. The print sheet is then gripped by the folding rollers and pulled into the folding gap, formed in-between the rollers and, in the process, is folded and also compressed along the folding line. The print sheet folded in this way is subsequently conveyed away in an downward direction with the aid of the folding rollers. Furthermore known are folding machines having a folding sword arranged below the folding table and having folding rollers arranged above the folding table. With these machines, the folded print sheet is respectively conveyed away in upward direction.
Regardless of its specific arrangement, a mechanical sword of this type requires very precise and involved geometric adjustments. Adapting the length of the folding sword to the format of the print sheets can furthermore be realized only with great expenditure. In addition, a mechanical folding sword requires a relatively large structural area, but is nevertheless hard to access. Owing to the required high folding capacity, the folding sword must move at the highest possible speed and therefore impacts at relatively high speed with the print sheet. To avoid contact with the folding rollers and to clear the space on the folding table as quickly as possible for supplying the following print sheet, the folding sword must reverse its movement direction just prior to reaching the folding rollers, so as to move in the direction opposite to the previous lowering movement. The print sheet to be folded is therefore in an undefined movement shortly before takeover through the folding rollers. The print sheet can thus deviate from the specified movement path and may not be gripped by the folding rollers, thereby resulting in the continuous danger of jamming of the successively supplied print sheets. In addition, print sheets arriving while offset, relative to the folding position, can only be folded with high technical expenditure along the predetermined folding line. Finally, the forming of so-called dog ears, meaning the folding over of the ends of the print sheets at high speeds, can hardly be controlled because corresponding guide elements are only conditionally usable owing to the space required for the mechanical sword. Accordingly, the danger of reducing the quality of the later printed product can be reduced with high technical expenditure, but cannot be eliminated.
When using an also known rotating folding sword, which is suitable for high folding capacities, the folding point in time can hardly be varied because of the mass inertia of the apparatus.
German patent document DE 10238502 A1 discloses a suitable method and apparatus for the cross-folding or the longitudinal folding of respectively one print sheet, wherein a pneumatic sword is used instead of a mechanical one, meaning a compressed air device consisting of a tube with preferably downward directed exit openings. The print sheet is supplied to this apparatus while positioned in a guide plane and is thus made available in a folding position where the predetermined folding line is located below the exit openings of the compressed air device. At that location, a control unit transmits a trigger pulse to the compressed air device for issuing a strong blast of compressed air, e.g. measuring several hundred bars, which blast is directed toward the folding line of the print sheet. As a result of the effect of this compressed-air blast, the print sheet is guided along its folding line into the folding gap between the folding rollers. Since a non-defined movement path of the print sheet is precluded in this way, the danger of a lower quality of the final printed product can thus be reduced as compared to using a mechanical sword.
Owing to the arrangement and the design of the compressed air device and the strong compressed air flow generated therewith, however, this solution does not permit an adaptation to changed properties for print sheets to be folded successively. These changed properties for the print sheets can include, for example, the format, the number of printed pages on each print sheet, as well as the grammage [grams per square meter] of the material used for the respective print sheet. For example, the strong blast of compressed air directed toward relatively lightweight print sheets, meaning print sheets with a small format and/or made of thin paper, could scrunch up these sheets before blowing them into the folding rollers. On the other hand, with relatively heavy print sheets, the blast of compressed air may not be sufficient to transport these sheets quickly enough to the folding rollers.
When creating a cross fold immediately prior to creating a longitudinal fold, the cross folding operation will create a gap between the folded print sheets, with the mechanical as well as with the pneumatic sword, wherein this gap corresponds to the length of the respective print sheet. When using a variable format, meaning sheets having a variable length which are to be folded successively, the relatively stable folding edge of the cross fold cannot be moved to a fixed position that is defined as advantageous for transferring the print sheet to the following machine because the folding edge of a following, shorter print sheet is otherwise already located in the sword folding region, thus making it impossible to fold the preceding print sheet. As a result, an involved and adjustable end stop is necessary. In addition, the lowered mechanical sword as well as the compressed air flow of the pneumatic sword, which is directed onto the print sheet to be folded, prevents an immediate feeding of the following print sheet and thus a quicker production.
Especially when processing further sheets that are printed sequentially with the aid of digital printers, which can print the sheets in the predetermined sequence for the finished printed product, thus making it possible to produce the product in relatively small piece numbers up to a single copy, successively following print sheets frequently have different properties in contrast to the traditional printing methods, such as the offset printing. The different properties can involve the format, the number of pages printed on each print sheet, the grammage and the porosity of the material used, the frictional values of the material surface, the residual moisture with or without upstream connected dryer and/or humidifier, the weight distribution and the frictional value distribution for the print sheet, relative to its folding line, the color assignment values, the electrostatic charge, as well as the temperature and moisture values of the material. Of course, the properties of the print sheets to be further processed are also influenced by changing processing and/or environmental conditions, such as the respectively used printing methods, the use of upstream-connected dryers and/or humidifiers, or the temperature and humidity in the production room. The properties of the print sheets as well as the processing and environmental conditions can either be detected during the further processing operation or can be taken from a database made available by a super-imposed system.
Finally, digital printers which transfer the print image directly from a computer to the printer and without the use of static print forms, are nowadays used to imprint higher and higher numbers of print material per time unit, which poses clearly higher requirements for the quality as well as the capacity of the devices used for the processing, for example the folding devices.